Exhaust gases from internal combustion engines contain nitrogen oxides NOx levels which are substantially larger than legislatively allowed levels. To reduce nitrogen oxides NOx emissions some kind of nitrogen oxides NOx aftertreatment is required. The two most feasible technologies are nitrogen oxide storage catalytic converter (LNT—Lean NOx Trap) and selective catalytic converters, so-called SCR (Selective Catalytic Reduction) both of which are described below in greater detail. A disadvantage of using LNT is the need for local rich (λ<1) (e.g. fuel is added to the air-fuel mixture) conditions to provide an oxygen poor environment for reduction of nitrogen oxides NOx. SCR additionally uses an additive to produce environmental conditions appropriate for reducing nitrogen oxides NOx. In the case of SCR, an ammonia NH3 compound is commonly added to produce a reducing environment. The present disclosure describes a system and method for nitrogen oxides NOx aftertreatment which avoids both the rich operation of an engine cylinder and the addition of an ammonia NH3 compound.
The disclosure relates to an internal combustion engine having at least one cylinder which has at least one outlet opening for the discharge of the exhaust gases via the exhaust-gas discharge system, in which:                each outlet opening is adjoined by an exhaust line, and        a storage catalytic converter for the reduction of the nitrogen oxides in the exhaust gas is arranged in the exhaust-gas discharge system.        
The disclosure also relates to a method for operating an internal combustion engine of said type.
An internal combustion engine of the above-stated type is used for example as a drive for a motor vehicle. Within the context of the present disclosure, the expression “internal combustion engine” encompasses in particular diesel engines, but also applied-ignition engines and also hybrid internal combustion engines which operate with a hybrid combustion process.
According to known methods to reduce pollutant emissions, internal combustion engines are equipped with various exhaust-gas aftertreatment systems. Even without additional measures, oxidation of the unburned hydrocarbons (HC) and of carbon monoxide (CO) takes place during the expansion and discharge of the cylinder charge at a sufficiently high temperature level, and in the presence of sufficiently large oxygen quantities. However, special reactors and/or filters must generally be provided in the exhaust tract in order to reduce the pollutant emissions under all operating conditions.
In applied-ignition engines, catalytic reactors are used which, through the use of catalytic materials increase the rate of certain reactions, ensure oxidation of HC and CO even at low temperatures. If nitrogen oxides NOx are also to be reduced, this may be achieved through the use of a three-way catalytic converter. However, use of a three-way catalytic converter requires stoichiometric operation (λ≈1) of the applied-ignition engine within narrow limits. Here, the nitrogen oxides NOx are reduced by means of the non-oxidized exhaust-gas components, specifically the carbon monoxides and the unburned hydrocarbons, wherein said exhaust-gas components are oxidized at the same time.
In internal combustion engines which are operated with an excess of air, for example diesel engines, but also applied-ignition engines which operate in the lean-burn mode or direct-injection applied-ignition engines, the nitrogen oxides NOx in the exhaust gas cannot be reduced owing to the lack of a reducing agent.
To reduce the nitrogen oxides, use is therefore made of selective catalytic converters, so-called SCR (Selective Catalytic Reduction) catalytic converters, in which a reducing agent is purposely introduced into the exhaust gas in order to selectively reduce the nitrogen oxides. SCR catalytic converters use ammonia and urea as reducing agents. However, SCR catalytic converters may also use unburned hydrocarbons as a reducing agent. The latter is also referred to as HC enrichment, wherein the unburned hydrocarbons can be introduced directly into the exhaust-gas discharge system. Additionally, unburned hydrocarbons can also be supplied by means of engine-internal measures, for example by means of a post-injection of additional fuel into the combustion chamber after the actual combustion. In the present disclosure the post-injected fuel should not be ignited in the combustion chamber by the main combustion but rather should be introduced into the exhaust-gas discharge system during the charge exchange.
The nitrogen oxide emissions may also be reduced by means of a nitrogen oxide storage catalytic converter (LNT—Lean NOx Trap). In using an LNT, the nitrogen oxides are absorbed initially, during a lean-burn mode of the internal combustion engine. That is to say the nitrogen oxides are collected and stored in the catalytic converter before being reduced during a regeneration phase. In a regeneration phase nitrogen oxides are reduced by means of substoichiometric operation (for example, λ<0.95) of the internal combustion engine. With a lack of oxygen the unburned hydrocarbons in the exhaust gas serve as reducing agents. Further, engine-internal possibilities for the reducing agent enrichment of the exhaust gas, in particular of unburned hydrocarbons, are exhaust-gas recirculation (EGR) and, in the case of diesel engines, throttling in the intake tract. As stated above, with regard to SCR catalytic converters, an enrichment of the exhaust gas with unburned hydrocarbons may also be realized by means of a post-injection of fuel, which may also be regarded as an engine-internal measure. A disadvantage of said approach is oil thinning. It is possible to dispense with engine-internal measures if the reducing agent is introduced directly into the exhaust-gas discharge system, for example by means of an injection of additional fuel upstream of the LNT as is an object of the present disclosure.
During the regeneration phase, the nitrogen oxides NOx are released and converted substantially into nitrogen dioxide N2, carbon dioxide CO2 and water H2O. The temperature of the storage catalytic converter should preferably lie in a temperature window between 200° C. and 450° C., such that a fast reduction is ensured and no desorption without conversion of the re-released nitrogen oxides NOx takes place. Such effects may be triggered by excessively high temperatures.
One difficulty in the use of an LNT arises from the sulfur contained in the exhaust gas. The sulfur is absorbed in the LNT and must be regularly removed by means of desulfurization. For this purpose, the LNT must be heated to high temperatures, conventionally between 600° C. and 700° C., and supplied with a reducing agent. Owing to the high temperatures required for the desulfurization, it is expedient for the storage catalytic converter to be positioned in as close-coupled a manner as possible, that is to say as close as possible to the outlet of the internal combustion engine, such that the exhaust gases are given little distance and time to cool down.
As described in detail above, both exhaust-gas aftertreatment systems, the SCR catalytic converter and the storage catalytic converter (LNT), require a reducing agent for the reduction of the nitrogen oxides NOx.
The use of fuel, that is to say of unburned hydrocarbons, as reducing agent injected into the combustion chamber opposes the basic aim of minimizing fuel consumption and improving the efficiency of the internal combustion engine. Furthermore, with increasing fuel consumption, the emissions of carbon dioxide CO2 also increase.
The use of ammonia as reducing agent requires the provision of an additional vessel for storing the ammonia, or urea from which ammonia NH3 is formed during the course of a thermolytic reaction.
Both types of reducing agent strategy, burning rich, and injection of ammonia NH3, constitute operating liquids which are consumed, entail costs and are harmful to the environment to a greater or lesser extent.
For the stated reasons, it is therefore advantageous to use as little reducing agent as possible, which may also be achieved through the most effective possible utilization of the reducing agent which is used. The present disclosure details an internal combustion engine having at least one cylinder which has at least one outlet opening for discharge of exhaust gases via an exhaust gas discharge system, comprising: an exhaust line adjoining each outlet opening; and a storage catalytic converter for reduction of nitrogen oxides in the exhaust gas is arranged in the exhaust-gas discharge system; wherein a bypass line for bypassing the storage catalytic converter is provided, the bypass line branches off from the exhaust-gas discharge system upstream of the storage catalytic converter so as to form a first junction and opens into the exhaust-gas discharge system again downstream of the storage catalytic converter so as to form a second junction; a device for introducing reducing agent into the exhaust-gas discharge system being provided between the first junction and the storage catalytic converter, and a shut-off element is provided for controlling the exhaust-gas mass flow conducted through the storage catalytic converter.
The present disclosure provides an exhaust gas bypass line which bypasses the storage catalytic converter. Conducting exhaust gas flow away from the storage catalytic converter reduces air flow to the converter during cleaning. Reducing air flow in the storage catalytic converter during cleaning reduces oxygen available to the storage catalytic converter. With the addition of a reducing agent, in this case fuel from the device for introducing reducing agent, into the exhaust line upstream of the storage catalytic converter, in addition to reducing available oxygen, conditions are improved for reducing nitrogen oxides NOx. Furthermore, limiting exhaust gas flow during cleaning limits air velocity through the storage catalytic converter further improving conditions for reduction of nitrogen oxides NOx. Injection of fuel into the exhaust-gas discharge system directly is advantageous over rich operation of the engine as the method of the present disclosure can be performed at any engine speed or load. Furthermore, unburned hydrocarbons HC passed through the storage catalytic converter during cleaning may serve as a reducing agent to an SCR downstream of the catalytic converter.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.